In the production of paper products from recycled paper, ink removal (de-inking) is critical if the brightness and cleanliness of the resulting product is to meet the standards of the end user. The most common way to effect de-inking is by flotation in which a slurry of ink laden recycled pulp is exposed to a source of minute air bubbles. The hydrophobic ink particles tend to adhere to the air bubbles and rise to the surface of the slurry and form a froth. The ink laden froth is removed leaving a recycled de-inked fiber that can be used to produce recycled paper.
One typical way of de-inking is the use of gas sparged hydrocyclones such as shown in U.S. Pat. Nos. 5,069,751, 5,173,177 and 5,131,980, the disclosures of which are hereby incorporated by reference herein. In these systems pressurized air passes radially through a porous cylinder and contacts the swirling slurry. Air bubbles collide with and attach to the ink particles in the liquid slurry and pass through the slurry and float in the form of a column to the top of the cyclone where they are removed. The slurry, with ink removed, continues down the cyclone and passes out of the base of the unit. At the base, an orifice, restriction, or "pedestal" is provided which provides a support for the foam column and a restriction to slurry flow. The amount of flow out of the rejects outlet is controlled by, among other things, adjustment of this restriction. Such hydrocyclones (such as sold commercially by Kamyr, Inc. of Glens Falls, N.Y. under the trademark "GSC.RTM.") function as a de-inker per se. While such devices provide an effective means for removing contaminants from pulp slurries, the present invention provides an improved method and apparatus for removing contaminants while utilizing such a hydrocyclone.
According to the present invention a method and apparatus are provided which utilizes the unique performance of the above hydrocyclone in cooperation with other equipment to provide an improved method of removing contaminants, e.g. ink particles, from recycled pulp slurries. According to the present invention hydrocyclones, such as described above, are operated to function as a "contactor" rather than a de-inker per se, though de-inking may also occur. With such hydrocyclone contactors connected to a flotation (or other type of separation) vessel an improved de-inking apparatus is provided. One advantage of this hydrocyclone contactor compared to conventional de-inking systems is that it can handle large increases in air flow without disrupting the downstream operations. With large air volume increases, any air that does not attach to the contaminants simply passes through the slurry vortex and is discharged through the rejects outlet at the top of the hydrocyclone. Thus large volumes of air can be introduced into the contactor and contaminant contact or removal increased without adversely affecting the operation of the system. This is particularly effective in treating heavily ink or contaminant laden slurries.
The hydrocyclones that are utilized according to the present invention are substantially identical to the conventional GSC.RTM. hydrocyclones (and those shown in U.S. Pat. Nos. 5,069,751 and 5,131,980) except the restriction at the outlet of the hydrocyclone is eliminated, or at least has a different function. Where in conventional GSC.RTM. hydrocyclones the role of the restriction, or pedestal, was to support the foam column, the restriction in the contacting hydrocyclone, if present, functions to regulate reject flow out the top of the unit.
According to the present invention the hydrocyclone is tolerant of (and sensitive to) gross changes in air flow. However the actual air flow to the hydrocyclones according to the invention is less than in the conventional hydrocyclones as described above. For example the air flows to the hydrocyclone according to the invention are approximately two-thirds less than the air flow to a conventional, de-inking GSC.RTM. hydrocyclone.
According to a first aspect of the present invention a method of de-inking a slurry of recycled cellulose fibers is provided which comprises the following steps: (a) Introducing an ink-laden slurry of recycled cellulose fibers into the first end of a vortex having first and second opposite ends. (b) Sparging gas radially inwardly into the slurry to cause gas bubbles to attach to contaminants and ink particles to form an aerated slurry. (c) Discharging a substantial portion of the aerated slurry from adjacent the second end of the vortex, and into a separation vessel. (d) Effecting separation of a foam of ink and contaminant-laden gas bubbles from the cellulose fiber slurry in the separation vessel. And, (e) removing from the separation vessel the foam with associated ink and contaminants in a first stream, and de-inked cellulose fiber slurry in a second stream.
Typically the separation vessel is a foam separation vessel and steps (d) and (e) are practiced by flotation. The flotation vessel typically has a liquid level, and step (c) is practiced to introduce the slurry below the liquid level in the flotation vessel. There is also then the further step of controlling the rate of flow out of the vortex by varying the liquid level in the flotation vessel, which is particularly effective in diverting extremely dirty furnish out of the rejects outlet by raising the level in the flotation column.
Steps (a) through (c) are typically practiced with the slurry having a consistency of about 0.5-2.0% solids (optimum normally about 1%), and step (b) is practiced with a gas to slurry volume ratio of about 0.1 to 1.0 to 1, preferably 0.2 to 0.7 to 1, typically, 0.3 to 0.4 to 1. The vortex (defined by a hydrocyclone) typically has a rejects flow from the first end thereof, and there is the further step of controlling the flow of rejects from the first end of the vortex so that it is about 0-20% (typically about 8-12% when rejects are withdrawn) of the liquid volume of slurry flowing through the vortex.
The flotation vessel may have a curved bottom and an accepts outlet in the curved bottom and a liquid surface. In this situation step (c) is typically practiced so as to introduce slurry into the flotation vessel in a direction away from the accepts outlet so that the slurry flows in part along the curved bottom and up toward the surface of liquid in the flotation vessel before passing to the accepts outlet (which may be covered in part by a baffle plate).
According to another aspect of the present invention a de-inking system is provided comprising the following components: A gas sparged hydrocyclone having first and second ends, a slurry inlet adjacent the first end, and a slurry outlet adjacent the second end. A flotation vessel having a liquid level therein, a top, and a bottom. A conduit connecting the slurry outlet to the flotation vessel and having a discharge opening below the liquid level in the flotation vessel. A slurry accepts outlet from the flotation vessel bottom.
This system may also include a means for removing foam at the liquid level, with ink particles and contaminants therein, from the flotation vessel. This foam removing means may comprise a wide variety of individual components or combination of components. For example the foam removing means may include a rotating mechanical element, which may rotate about either a horizontal or vertical axis and may have blades, paddles, or like elements with a wide variety of shapes.
The flotation vessel bottom may be curved and the slurry accepts outlet disposed in the curved bottom, and covered in part by a baffle plate. The conduit may include a header having openings therein, with the openings directed away from the slurry accepts outlet. Alternatively the flotation vessel may be tapered so that it has a larger cross-sectional area at the top than at the bottom, and the bottom may be flat.
The hydrocyclone may include a rejects outlet adjacent the first end thereof, and the system may further comprise a rejects tank operatively connected to the foam removing means with a second conduit extending from the hydrocyclone rejects outlet to the rejects tank. The hydrocyclone has an open, substantially unrestricted second end in open communication with the conduit so that it acts as a contactor rather than a de-inker per se, in the preferred embodiment, although a restriction may be provided instead and the system operated so that only a relatively small amount of rejects are withdrawn through the hydrocyclone rejects outlet.
It is the primary object of the present invention to provide an effective method and apparatus for high efficiency de-inking of recycled cellulose fiber slurries, to produce high quality recycled paper products. This and other objects of invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.